Microsoft hyped the release of Internet Explorer 9 as much as it could, but it wasn't enough to stop the juggernaut of Firefox 4. Within days, it was clear Mozilla won the war to get the most user downloads.

But Microsoft complained that the comparison wasn't really fair and it has a point. IE9 is only for Windows 7 (well, Vista too), while Firefox works on any operating system whether it's Windows, Mac or Linux. That makes a big difference since more than half of computer users still run Windows XP.

Some new numbers out today from market share tracker Net Applications are helping Microsoft argue its case that IE9 will be the dominant browser on Windows 7. And - most crucially - Microsoft argues that IE9 is special because it is designed for the users of one operating system, and not for the "lowest common denominator." Windows XP isn't good enough for IE9, Microsoft says.

Google's latest version of Chrome is already up to 10.19% on Windows 7. Firefox 3.6 holds 19.53% of the market for Windows 7 users and IE8 is at the top with 51.57%. Microsoft has sold more than 300 million copies of Windows 7.

Among the three major browsers released in the past month, Chrome 10, IE9 and Firefox 4, Chrome surged out to the early lead on Windows 7 in large part due to Chrome's auto-update feature. Microsoft is not only limiting IE9 to Windows 7 and Vista, but it hasn't flipped the automatic update switch for most users yet. That'll happen at the end of June, says IE senior director Ryan Gavin.

Once automatic updates are turned on for everyone, Gavin expects somewhere between 40% and 60% of Windows 7 users to upgrade from IE8 to IE9. (You can decline the update.) Currently, users have to make the effort to download IE9 themselves because Windows Update has only been turned on for those users who had already downloaded the beta or release candidate versions of IE9.

Long ago, Microsoft got itself into trouble by bundling Internet Explorer with Windows and finding ways to exclude competing browsers. Microsoft has lost tons of market share since settling with the U.S. government, and by limiting IE9 to its newest operating systems Microsoft is actually diminishing its own market share on purpose. (IE's total share across all versions dropped from 56.77% to 55.92% between February and March. Firefox, Chrome and Safari all gained share.)

But Gavin argues that's ok. Microsoft is taking the somewhat unique view that a browser should be tailor-made to one operating system, so as to avoid catering to the "lowest common denominator" and thus diminishing user experience for all. IE9, designed for Windows 7, is the only browser to take advantage of its unique hardware acceleration features and will thus offer "immersive experiences" that aren't possible on Windows XP, Gavin says.

"In the future, the browser is only as good as the operating system and the device it runs on," Gavin says. "We have to think about these things as being integrated."

Browsers that work across different operating systems have to be built with a series of tradeoffs, he argued. Microsoft doesn't intend to build a browser for Macs, because it's not necessary. "There are a great set of browsers out there across different platforms," Gavin said.

Coincidentally, I was speaking with Linux Foundation executive director Jim Zemlin this week, and he made the opposite point. The rise of the browser, he says, is taking focus away from the operating system and thus bulky software like Windows is becoming less crucial. In the future, all you'll need is a browser with a lightweight OS running underneath, he says.

In my own Web surfing experiences, I find IE9 to be a very well organized browser, and quick, but simply not as fast as Google Chrome, which I use on both Windows and Mac. (See also: First look at Google Chrome 10.)

Microsoft argues that the IE9/Windows 7 combination, with the aforementioned hardware acceleration, will allow website developers to build incredible applications for Microsoft's browser that won't be possible on competing platforms. It's early yet, so we'll give Microsoft some time to make good on that promise.